1. Field of the Invention
This invention is directed generally to the field of devices for robotically centering components having a surface pattern such as die substrates having a circuit pattern or ball grid array. More particularly, the present invention relates to a method and apparatus for providing offset information to a standard component in-flight registration and orientation system, thereby allowing the accurate placement of a surface pattern of the component for use with either standard "die-up" or flip-chip assembly methods.
2. Background Art
In electronic circuitry manufacturing, automatic placement machines for moving discrete electronic components or semiconductor die (components) from a source location to a target placement area are known in the art. For example, one commonly known type of automatic placement machine includes a mechanical transport such as a pick and place machine. It typically has a pick-up arm capable of movement in two or three dimensions and has a vacuum nozzle for providing a component pick up function. The pick and place machine is used to pick up discrete electronic components such as resistors, capacitors, integrated circuits and the like from a component supply source (e.g., component supply feeders, magazines, and component bins) and place the components on a printed circuit board or on a substrate, collectively known as workpieces. Examples of a typical workpiece include a semiconductor die that has a surface pattern or geometry, a printed circuit board that has electrically conductive traces, a plurality of pads to receive surface mounted components, or a ball grid array. The surface patterns have small dimensions which are getting smaller as the market demands greater component densities. Consequently, it is of importance that the position of each discrete component to be placed as well as the relative location of the workpiece be known to a degree of accuracy suited for such dimensions before placement.
A typical pick and place machine, however, suffers from component placement deviations due to various causes such as mechanical backlash and component movement during the pick-up process. Consequently, various methods and apparatuses have been devised to compensate or correct for such placement deviations.
Providing precise placement of a component using a pick and place system requires the ability to accurately determine the actual position of the component relative to a reference position before the placement of the component at a target placement area. A typical approach includes combining a pick and place machine with a device for measuring the actual position of the component relative to a reference location after the component is picked up but before placement at a target placement area. This approach relies on occluded topography such as the outer perimeter or external shape of a component.
A typical pick and place machine has a vacuum nozzle, known as a quill, that provides a grasping and pick-up function. While grasping the component but before placing the component at a target placement area, the pick and place machine determines the actual position of the component relative to the position of the quill (in-flight position). The in-flight position information is typically in the form of an X-Y coordinate relative to the position of the quill (X-Y deviation) and a theta value relative to a positioning axis such as the vertical, longitudinal axis of the quill. Note that since the quill is typically orthogonal to a grasping surface of the component such as a top surface, the theta value provides the angular deviation of the component relative to the axis of the quill.
By determining the actual position of the component relative to the position of the quill, i.e., the in-flight position of the component, the pick and place machine may determine whether the component is at a predetermined normative X-Y position and angular orientation ("expected position") and if not, compute a corrective offset that is used to correct any deviation of the component from the expected position while the pick and place machine is grasping the component but before placement of the component. By ensuring that the component is at the expected position before placement, precise and accurate placement at a target placement area may be achieved since the actual location of the component is known and may be modified to a predetermined in-flight position before placement.
One approach to obtaining the in-flight position of a component is described in U.S. Pat. No. 4,615,093 to Tews, et al. As seen in FIG. 1A, Tews picks up a component 110 with a pick and place machine 112 and brings the component 110 up to a position scanning station 114, which is mounted and carried with the pick up head of machine 112, so that the in-flight position of the component 110 may be obtained. Position scanning is carried out at any point between pick up and placement. The in-flight position includes the X-Y position of the component relative to a nominal position (the X-Y deviation) and an angular position relative to a positioning axis 116 (the rotational deviation). In FIG. 1B, the method of obtaining the in-flight position of a component 118 by scanning the external dimensions such as the perimeter edge 120 of the component 118 is shown. Specifically, scanning is achieved by using a light source such as a row 122 of individual beam emitting elements 124 or another source of light. Opposite to the light source is a row 126 of receiving elements 128 such as photo-diodes so that the shadow cast by the component 118 may be determined and its positioned determined thereby.
In FIG. 2 U.S. Pat. No. 5,278,634 to Skunes et al. refines the approach taken by Tews et al. by obtaining the in-flight position of a surface mount-type component 210 using a sensor system 212. The sensor system 212 is comprised of a light source 214 which is passed through a collimating lens 216 and then through an aperture 218 to create a wide beam or stripe of collimated light 220 which is focused past the body of the component 210 being aligned to strike a multi-element CCD sensory array 222. The sensor system 212 is mounted directly on the carrying mechanism for the surface mount component placement machine such as a pick and place machine. During transit of the component between the source of the component and the target placement area, the component is rotated forming a shadow on the detector array while the array is monitored. When the minimum width of the shadow is detected, the correct angular orientation is determined, the average of the edges of the shadow when compared with the center of the quill determines the coordinate (X, Y) location of the component on the quill. Two alignment operations normally occur, each displaced by 90 degrees. Thereafter, the sensor sends correcting signals to the component placement machine to assure the correct angular position and the correct X,Y position for the component to be placed at the target placement location by the component placement machine. Thus, Skunes et al. like Tews et al. relies on the external dimensions such as the perimeter edges of the component to produce the necessary shadows that are monitored by the sensor array.
U.S. Pat. No. 5,559,727 to Deley et al. discloses an approach that is very much like the approaches disclosed by Tews et. al. and Skunes et al. Deley et al. discloses combining a positioning sensing system with a mechanical transport which are mounted on a movable frame with each other. In FIG. 3, the position sensing system 310 includes projecting a wide beam of collimated light 312 on a component 314 whose position is to be determined. The unblocked rays of light 316 are received and processed by an intermediate series of lenses 318 and an aperture 320. The rays of light are then directed as an image onto a linear array sensor 322 having a plurality of high density pixel sensors whose position is known relative to the pick up tool. The component 314 whose position is to be determined is rotated in the imaging field. A computer/controller (not shown) is coupled to the linear array sensor and an encoder for producing sets of X, Y and theta data indicative of the refocused collimated light rays projected onto the linear array sensor 322 and the computer/controller is programmable for analyzing the digital data for determining the X, Y, and theta position of the component on the pick up tool. Deley et al. relies on the external dimensions or shape such as the perimeter edges of the component and/or component leads to produce a wave form representing the varying silhouette of the component when the component is rotated in front of the sensing system.
However, as will be described in more detail below, the above approaches to determining the in-flight position of the component are ill-suited for use with components that include semiconductor dies having high density surface patterns since they measure the external dimensions of the die, i.e., the perimeter edges rather than the position of the circuit pattern disposed on the die. Thus, any measured position obtained reflects the actual position of the die not the surface pattern disposed thereon.
As generally known, the use of semiconductor die with either die up or flip-chip assembly methods includes removing die having a surface geometry or pattern such as a circuit pattern from a source location and placing the die to a new location for subsequent assembly. Since semiconductor die as well as other types of small components having lithographic or printed patterns are typically produced in an arrayed manner on a wafer, removal of each die from the wafer includes the sawing or dicing of the wafer, thereby producing individual component die as known in the art. Once the sawing or dicing process has been completed, placement of each die typically includes using a mechanical transport such as the types described above to place each die at a new location for subsequent assembly.
Placement of the die is critical since any subsequent assembly to be done to the die requires knowledge of the actual position, i.e., the X-Y position and angular orientation, of the circuit pattern on the die. For example, a subsequent assembly after dicing may include wire bonding the interconnects located within the circuit pattern. A process that inherently requires knowledge of the actual position of the circuit pattern in order to obtain the precise location of the interconnects requiring wire bonding.
As disclosed by Tews et al., Skunes et al., and Deley et al., the actual position of the component is calculated by measuring the external features of the component that provide an occluded topography such as the leads or package body of a discrete component. The same approach may be used for semiconductor die where the occluded topography used is the perimeter edges of the die. The perimeter edges of the die are used to obtain the actual position of the die. The actual position of the die is used in turn to calculate the X-Y position and angular orientation ("extrapolated position") of the circuit pattern using the centroid, i.e., mean center, of the circuit pattern as a point of reference. The extrapolated position of the centroid of the circuit pattern is not measured but assumed to be at a constant offset from the actual position of the perimeter edges of the die.
However, relying on the perimeter edges of a die to extrapolate the position of the circuit pattern is not well-suited to semiconductor die having very high surface pattern densities. Such densities leave very little room for variances between the actual location of the die pattern and the perimeter edges of the die. Specifically, although the pattern on the surface of a die is highly accurate and may be ensured to vary less than 0.5 microns as currently known in the art, the perimeter edges are less accurate and vary up to 25 microns.
The position of the perimeter of semiconductor die varies from die to die because typically more than one die are produced from a single silicon wafer and must each be sawed to produce an individual die. But the dicing or sawing process has a limited best accuracy of being greater than 25 microns. Due to factors such as the wearing of the dicing saw and set-up inaccuracies, the sawed edges comprising the perimeter of each die are typically off-center and vary relative to the centroid of the circuit pattern on the surface of the die. As die features become smaller and denser in new semiconductor designs, the accuracy needed in automatic assembly systems has increased.
Consequently, using the perimeter of the die as an accurate and consistent reference to the centroid position and orientation of the die surface pattern is ill-suited for placement purposes for such dies.
Also, another approach as known in the art is to measure the circuit pattern rather than using the perimeter edge of the die prior to picking up the die with a mechanical transport such as a pick and place system. However, placement is carried out "blind" and without regard to any errors introduced by the mechanics of picking-up the die ("pick-up errors"). These pick-up errors affect placement accuracy of the circuit pattern disposed on the die and must be compensated for in a later step of the assembly process such as through wire-bonder offset correction.